Human thermal comfort is a combination of a subjective sensation (how we feel) and several objective interaction with the environment (heat and mass transfer rates) regulated by the brain. Comfort depends on several physical magnitudes that we may group as:

• Person-related. Deep body temperature in humans is always close to 37 ºC independent of environmental temperature, as first measured in the 1660s by Boyle. It may depart a few degrees under unhealthy circumstances, particularly above that value, as with fever, or during heavy prolonged physical exercise. We must continuously evacuate heat through our skin to the ambient to compensate our metabolic dissipation, with a baseline rate of about 1 W/kg, increasing with physical activity up to 5 W/kg; e.g. it is around 100 W for an adult in office-work. Skin temperature is usually below 33 ºC, allowing the heat outflow, but it depends a lot on external conditions, clothing, and actual and previous activity levels. Besides, age and risk groups (babies, elders, ill persons), previous accommodation (e.g. changing from indoors to outdoors), habits (e.g. clothing difference among seasons and sex), personal preferences (some people feel comfortable cold or hot), and actual mood (the state of mind, feeling happy or nervous) may have an influence (comfort is not just a physiological problem but psychological too).

• Environment-related. Air temperature (or water temperature if diving), background radiant temperature (of walls, sky, and Sun, if any), air relative humidity, and wind speed. And not only average values matter, but their gradients and transients too. Non-thermal environmental variables like ambient light and noise may affect the thermal sensation too. The most difficult to measure of the parameters governing thermal comfort is the background radiant temperature, which depends on direct solar irradiance, wall solar reflectance (albedo), sky temperature, wall temperature, and all the geometric view factors involved

There has been a tendency to combine all environmental variables in an effective or apparent temperature, and all personal response in a few degrees of comfort (or discomfort); the 7-scale thermal feeling is:

• Uncomfortable cold, when >95% of people in a significant group complain of being cold.

• Cool, or bearable cold, when some 75% of people in a significant group complain of being cold